Many types of equipment and industrial processes require accurate control of the flow of liquid and gaseous fluids over a broad range of fluid pressure and flow rates. It is particularly important that flow controllers for such equipment and processes be able to accurately meter fluid flow over a broad range of flow rates. One problem with many prior art valves is that the amount of torque required to adjust the valve to alter the flow rate increases proportionally to fluid pressure within the valve. This is because the fluid pressure is operating in the valve components that must be rotated during valve adjustment. As a result, adjustment can be difficult to accomplish at high pressures.
Many conventional flow control valves utilize a piston carrying a needle that acts against a spring set contained between the piston and the bottom wall of the valve chamber in which the valve seat is formed. To adjust the valve, the bias force of the piston spring set can only be adjusted from an external source, or an adjustable valve seat has to be provided. An improvement on such valves is provided in the metering valve of U.S. Pat. No. 5,427,139 to Hilton, the disclosure of which is hereby incorporated by reference. This valve overcomes some shortcomings of conventional valves by including an adjustable floating piston and needle assembly that provides for adjustment against internal differential pressure independent of the total system pressure.
While representing an advance in adjustability and metering accuracy, the Hilton '139 metering valve maintains its highest degree of accuracy of flow regulation only in a narrow range. Hardware modifications are required to operate in other ranges with the same degree of accuracy.
An industry-recognized measure of the ability of a valve to control flow rate over a range of flow rates is the "turndown ratio". Thus, if a valve has a "turndown ratio" of 10:1 and is capable of accurately regulating a maximum flow rate of 100 liters per hour, then the smallest flow rate that the valve can accurately regulate is 10 liters per hour (i.e., one tenth of the maximum flow rate). Typically, the "turndown ratio" of prior art valves is from about 4:1 to about 10:1. Thus, there is a need for a valve that can accurately regulate a wide range of fluid flow rates and that requires a relatively small amount of torque to adjust the flow rate of the valve during operation.
It is also desired to have a metering valve that is not unduly sensitive to pressure fluctuations in the system. This is particularly the case for downstream pressure fluctuations that may induce a sinusoidal flow rate fluctuations in conventional valves.